Entity schema validation test

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(Prompted by @rspeicher reviewing in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15515#note_215738264)

https://gitlab.com/gitlab-org/gitlab-ee/blob/cd82f261ba6339226da03794c3315e7fffaded6d/app%2Fserializers%2FREADME.md#how-to-write-tests-for-a-serializer mentions to validate using match_response_schema (sic, match_schema ?)

Does it make sense to repeat a similar test using match_schema on the Entity as well ?

https://gitlab.com/gitlab-org/gitlab-ee/blob/cd82f261ba6339226da03794c3315e7fffaded6d/app%2Fserializers%2FREADME.md#best-practices that we should test entity as well, but how exactly is not clarified, so here I am.